This invention relates to an improvement of an optical pickup for detecting focussing errors by a knife edge test.
A conventional optical system of an optical pickup of is illustrated in FIG. 5.
As is shown in FIG. 5, optical system A includes a laser beam element 1 for emitting a beam, a collimate lens 2 for making the beam in parallel pencil rays, a polarizing beam splitter 3, a quarter wavelength plate 4, an objective lens 5 for forming a spot on an optical disk D, a condenser lens 6 for converging a reflected beam, a light receiving element 7 for detecting focussing errors, a knife edge 8 disposed between the condenser lens 6 and the light receiving element 7 for detecting focussing errors to block generally a half portion of the beam in order to detect focussing errors by a knife edge test, and a half mirror prism 10 that is disposed between the knife edge 8 and condenser lens 6 so as to radiate a portion of the reflected beam to a light receiving element 9 for detecting tracking errors.
In the optical system A having the above-mentioned constitution, the beam emitted by the laser beam 1 and reflected by the optical disk D is converged toward the light receiving element 7 for detecting focussing errors by the condenser lens 6 and then split into two portions by the half mirror prism 10.
The beam passed through the half mirror prism 10 is blocked generally a half portion thereof by the knife edge 8. The remaining portion of the beam, which is not blocked by the knife edge 8, radiates the light receiving element 7 for detecting focussing errors. On the other hand, the beam reflected by the half mirror prism 10 is permitted to directly radiate the light receiving element 9 for detecting tracking errors. Each of the light receiving elements 7 and 9 outputs an error signal depending on the light receiving state thereof.
The light receiving element 7 for detecting focussing errors, as shown in FIG. 7, is a two-division element which is disposed in a converging position of the reflected beam during the focussing time of the objective lens 5.
Due to the above-mentioned arrangement, the reflected beam radiates an upper light receiving area 7a, as shown in FIG. 6, when the distance between the optical disk D and the objective lens 5 is too near, whereas it radiates a lower light receiving area 7b, as shown in FIG. 7, when the distance is too far. When focussing, the reflected beam is converged between the upper and lower light receiving areas as shown in FIG. 8.
However, in such a conventional optical system A of an optical pickup as described above, since the light receiving element for detecting focussing errors and the other light receiving element for detecting tracking errors must be disposed in independent positions and since a half mirror prism or half-transmitting prism is also required, the number of component parts becomes large and the optical system itself becomes bulky.
There is another conventional optical system which aims at solving the above-mentioned problems and attempts to detect both error signals by one light receiving element. One such example is shown in FIG. 9.
In this embodiment, a four-division light receiving element 11 illustrated is employed. A cylindrical lens 12 is disposed between the light receiving element 11 and condenser lens 6, for detecting focussing errors by an astigmatism method.
If an optical system is constructed in a way as mentioned above, it is true that both errors can be simultaneously detected by adding or deducting outputs of various light receiving areas a, b, c, and d of the light receiving element 11 [focussing error equals (a+d)-(b+c), and tracking error equals (a+c)-(b+d) ]. However, this optical system must be designed such that the beam constantly radiates the light receiving element in order to obtain a tracking error signal and a regenerating signal.
However, since a DRAW (direct read after write) type optical disk is designed such that the tracking error composition by a push-pull method becomes maximum, if the beam radiates the light receiving areas even during the focussing time, there arises another problem in that the tracking error composition contained in the beam interferes with the focussing error signal and, as a result, a correct focussing error detection is difficult to carry out.